Suitable visualization of the blood vessels plays a major role for numerous applications in medical imaging. This relates in particular to the identification of stenoses which may be caused by atherosclerotic lesions, such as calcification and/or soft plaque. It should be possible to identify and evaluation stenoses well in the scan images recorded from the patient. The known slice images which are obtained by multiplanar reformation (MPR) from a volume data record are suitable only to a restricted extent for this purpose since they generally each represent only a short section of a vessel which is currently located on the corresponding slice plane.
A further display technique which is used in this context, the technique of maximum intensity projection (MIP), admittedly makes it possible to produce a three-dimensional image impression of a relatively long section of the vessel profile, but likewise still does not provide a display for reliable identification of stenoses.
In order to allow stenoses to be identified better, it is known for so-called curved planar reformation (CPR) to be used, which results in slice images whose slice plane runs along the vessel axis. This makes it possible to identify the vessel lumen and possible anomalies of the vessel on this curved slice plane. However, stenoses may be formed eccentrically in the vessel cross section, so that the correct projection direction and perspective of the CPR images is very critical for identifying stenoses such as these. If this projection direction is not correct, then individual lesions may be underestimated, or even completely overlooked. It is therefore necessary to allow the CPR images to be rotated freely about the vessel axes in order to allow eccentric stenoses in images such as these to be reliably identified and evaluated as well. However, this is dependent on determining the three-dimensional profile of the vessel axis in the volume data record on which this is based.
At the moment, two methods for producing CPR images of a vessel are known to the applicant. In the first technique, a user interactively marks the vessel axis on images of a slice stack. For this purpose the user must interactively scroll through the stack of planar images, for example of an axial, coronal, sagittal or inclined image stack, and has to mark the vessel axis by way of a graphics input device, for example a mouse. The slice plane of the CPR image is then determined from the individual marking points by linking all of the marking points to a spline curve. Since all of the marking points are created and recorded only using a two-dimensional coordinate system, the image plane on the screen, only one CPR image is obtained for a single projection direction. A different slice stack of MPR images must be produced for renewed manual marking of the vessel axis for a further projection direction, corresponding to this further projection direction.
In the second known technique for producing CPR images, a completely automatic method is used to determine the three-dimensional vessel axis in the volume data record. However, this method requires segmentation of the vessel by way of a specific semi-automatic vessel segmentation algorithm. This technique is very time-consuming, since the segmentation of the vessel for determining the vessel axis must be carried out with high precision. After segmentation, the vessel axis can then be obtained fully automatically by forming a skeleton of the segmentation mask. The slice plane for the CPR image is then produced by projecting the three-dimensional vessel axis from a predeterminable projection direction onto a two-dimensional plane, with the slice plane of the CPR image running at right angles to this along the projected profile of the vessel axis. Since this technique results in the three-dimensional profile of the vessel axis, a different CPR image can be produced from a different projection direction at any time without having to determine the vessel profile again. This allows free rotation of the displayed CPR image about the vessel axis, so that eccentric stenoses can also reliably be identified. However, the method also requires time-consuming segmentation of the vessel under consideration.